Method for anodizing aluminum materials

ABSTRACT

Structural components made of aluminum and aluminum alloys are anodized in two sequential steps in two different electrolytes. The first electrolyte is a mixed acid of two inorganic acids such as a phosphoric-sulfuric acid mixture. The second electrolyte is a further acid mixture of an organic acid and an inorganic acid to form a tartaric sulfuric acid mixture. The two sequential anodizing steps result in a surface texture that has three excellent surface characteristics simultaneously, namely: a corrosion resistance, a coating acceptance for lacquer coatings and an adhesive bonding with other aluminum material components.

PRIORITY CLAIM

This application is based on and claims the priority under 35 U.S.C. §119 of German Patent Application 103 61 888.0, filed on Dec. 23, 2003, the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for anodizing aluminum materials, more specifically structural components made of aluminum and/or aluminum alloys.

BACKGROUND INFORMATION

It is known to anodize structural components made of aluminum materials including aluminum and aluminum alloys in two different sequential steps, whereby the respective different electrolytes comprise at least two components. The surface of the structural component is oxidized by applying a DC voltage to an electrolytic bath. At least one of the two electrolytes is a mixture of at least two inorganic acids forming an inorganic acid mixture.

The coating of structural components of aluminum materials with an anodized layer is intended to change the surface characteristics of these structural components. Anodized layers are porous layers which, depending on the respective production parameters of the coating method result in a different surface morphology or surface textures. For example, the purpose of an anodized layer is primarily to provide a corrosion protection of the basic material of which the structural component is made. A further purpose of the surface texture of such components is to provide a good adhesion strength if such components are to be adhesively bonded to one another, for example in the form of several plies. Yet another desirable characteristic of the surface texture is a good adhesion of lacquer coatings. So far, all three characteristics have not been achieved simultaneously without rather undesirable side effects.

The more important conventional anodizing methods for aluminum materials will now be described. The chromic acid anodizing (CAA) as described in German Industrial Standard Sheet DIN EN 3002 provides an anodized layer which has good corrosion resistance while also exhibiting useful adhesion characteristics as well as lacquer coating retaining characteristics. However, there is a serious drawback because the electrolyte in the anodizing bath contains chromate which is a carcinogenic. Therefore, even though the CAA method is currently widely used in aircraft construction, it will hardly be possible to keep using this method in the future.

The phosphoric acid anodizing (PAA) method is described in British Patent Publication GB 1,555,940. This type of anodization is specifically directed to the improvement of the adhesion characteristics of the components anodized by this method. U.S. Pat. No. 4,085,012 describes a similar method. The phosphoric acid anodizing (PAA) provides an anodized layer having a surface morphology or texture that is suitable for adhesively bonding components to each other and also function as a base coating for a subsequent application of a lacquer coating. However, a substantial drawback of an anodized layer produced by an electrolyte containing phosphoric acid, is seen in the rather limited corrosion protection characteristic.

Anodizing with an electrolyte containing phosphoric and sulfuric acids is described in a pamphlet TN-EVC 904/96. This type of anodization provides excellent adhesion characteristics for adhesive bonding as well as a base coating for lacquering. It is an advantage of this type of anodizing that chromates are not used. However, phosphoric sulfuric acid anodization produces coatings that do not provide a sufficient corrosion protection.

A so-called boric sulfuric acid anodizing (BSAA) is described in U.S. Pat. No. 4,894,127. Coatings produced by that method have a good corrosion resistance and provide a good base coating for the adhesion of lacquers. However, these coatings are not suitable for adhesive bonding with other components.

Yet another known method employs a DC current sulfuric acid anodizing which provides surface coatings having very high corrosion resistance. However, these coatings are neither suitable for adhesive bonding nor as a base coating for lacquering.

Yet another method is known as a so-called mixed acid anodization using as an electrolyte a mixture of tartaric acid and sulfuric acid (TSA) as described in European Patent Publication EP 1,233,084 A2. Anodized surfaces produced by that method have a good corrosion resistance and are suitable for a base coating for lacquering. However, the coatings are not suitable for adhesive bonding.

In view of the foregoing the only conventional anodizing method that achieves all three characteristics simultaneously is the chromic acid anodizing. However, in view of its use of carcinogenic chromates chromic acid anodizing will not be suitable for continued use. Other conventional anodizing methods using a single anodizing step satisfy at best two of the three requirements. In this connection reference is made to U.S. Pat. No. 5,486,283 which uses a so-called duplex method which employs a phosphoric-boric-sulfuric acid anodizing (PBSA). This PBSA anodizing is performed in a two step operation. The electrolytes used in both steps are inorganic electrolytes. The first anodizing step of the PBS method uses an inorganic acid while the second anodizing step uses an inorganic mixed acid. The coatings produced by the PBSA method have a good corrosion resistance and serve simultaneously as adhesion enhancing agents for lacquers and adhesive bonds so that this method satisfies all three requirements. However, there is still room for improvement.

OBJECTS OF THE INVENTION

In view of the foregoing it is the foregoing objects singly or in combination:

-   -   to provide a two step anodizing method that is particularly         suitable for improving aluminum or aluminum alloy surfaces and         which provides a surface texture with a good corrosion         resistance while simultaneously serving as a suitable base         coating for subsequent lacquering and as an adhesive bonding         base coating;     -   to provide a two step anodizing method that does not cause any         technical problems in the sequence of the anodizing steps while         simultaneously avoiding health problems;     -   to make it possible that two directly sequential anodizing steps         can be performed in an optimally short time;     -   to provide an anodizing method that is particularly suitable for         all aluminum materials that are used in aircraft construction,         particularly high strength aluminum alloys and weldable aluminum         alloys; and     -   to provide an anodizing method, the parameters of which are         readily adaptable to the requirements for achieving different         surface characteristics;

The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification. The attainment of these objects is, however, not a required limitation of the claimed invention.

SUMMARY OF THE INVENTION

The above objects have been achieved according to the invention by an anodizing method for surfaces of aluminum and aluminum alloys which method comprises the following steps:

-   a) first preparing a mixed inorganic acid as a first electrolyte, -   b) first exposing said surface to said first electrolyte to provide     an anodized first surface coating; -   c) second preparing a mixed acid of an organic acid and an inorganic     acid as a second electrolyte, and -   d) second exposing said surface to said second electrolyte to     provide a second surface coating.

Thus, the first anodizing step is performed with an inorganic acid mixture while the second anodizing step is performed with a mixture of an organic and an inorganic acid.

The present method is suitable for all aluminum alloys, particularly also for high strength aluminum alloys particularly including corrosion sensitive aluminum alloys. Another advantage of the invention is seen in that the materials used for preparing the electrolytes are neither carcinogenic nor toxic. Still another advantage of the invention is seen in that the individual parameters of the electrolytes are easily adapted to the required functional and textural characteristics of the coatings to be produced. These parameters can be individually adapted in each of the individual anodizing steps.

The present method produces in a two step operation an oxide film particularly on aluminum alloys as they are used in the aircraft construction. The first anodizing step produces a first layer having preferably a thickness within the range of 1 to 2 μm. This first layer has a substantial porosity which is exceptionally well suited for adhesively bonding aluminum components to one another. The second anodizing step forms a low porosity layer having preferably a thickness within the range of 2 to 4 μm underneath the first layer. Presumably, the second layer formed in the second anodizing bath can grow below the first layer due to the substantial porosity of the first layer and probably due to a certain affinity between the aluminum material and the second electrolyte both. Further, it is possible to reverse the step sequence. More specifically, to perform the second step first and the first step last. In both possibilities the highly corrosion resistant layer will be formed between the aluminum surface and the adhesion enhancing outer anodized layer. The corrosion resistance of the second anodized layer can be further improved by a subsequent densifying step, for example by a rolling or pressing operation. As mentioned above, the present two step operation is equally suitable for all aluminum alloys used in the aircraft manufacture, particularly high strength aluminum alloys of the series 2XXX and 7XXX as well as for the weldable aluminum alloys of the series 6XXX. The preferred thicknesses of the layers produced according to the invention are within the range of 1.5 to 10 μm for the individual layers.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood, it will now be described in connection with example embodiments thereof, with reference to the accompanying drawings, wherein:

FIG. 1 shows a schematic, sectional view of an aluminum component coated according to the invention; and

FIG. 2 shows an anodizing voltage diagram as a function of time.

DETAILED DESCRIPTION OF A PREFERRED EXAMPLE EMBODIMENT AND OF THE BEST MODE OF THE INVENTION

FIG. 1 shows an aluminum component 1 provided with a first anodized layer 2 and a second anodized layer 3. The layer thickness is exaggerated having regard to the fact that the thickness of the layers is within the range of a few microns, preferably in the range of 1 to 10 μm as mentioned above.

It will be noted that in FIG. 1 the anodized layer 2, which is produced first, appears as top layer having a very rugged surface 2A with deep pores 2B and roots 2C reaching into the second layer 3 shown to adhere to the surface 1A of the component 1. The second layer 3 is anodized after the first layer 2 has been anodized. Yet, the second layer 3 appears between the first layer 2 and the surface 1A. It is assumed that the second anodized layer 3 can grow through the deep pores 2B and then adhere to the surface 1A and to the first layer 2 due to affinities between the aluminum materials on the one hand and the two different electrolytes on the other hand. In this context it is possible to achieve the same results by first anodizing the second layer 3 on the surface 1A and then anodizing the first layer 2 on the top of the so-called second layer. However, bonding between the layers 2, 3 and the surface 1A appears to be better if layer 2 is anodized first to generate the shown intermeshing.

FIG. 2 illustrates the control of the DC anodizing voltage as a function of time. A ramp voltage RV rises within a maximum of 10 minutes to a maximum voltage which becomes a constant or plateau voltage PV which does not exceed 25 V maximally and which continues to a maximum of about 90 minutes. Further details how these parameters may differ for different anodizing requirements will be described below.

Prior to exposing an aluminum or aluminum alloy component to the two anodizing steps according to the invention, the surfaces of the component are conventionally cleaned in a grease removing bath and then in a pickling bath, whereby basic and/or acidic pickling steps may be performed. After such cleaning the aluminum components are exposed in sequence to two anodizing electrolyte baths according to the invention. A first electrolyte for the first bath is an inorganic acid mixture of a phosphoric-sulfuric acid and results in the application of a highly porous anodized layer 2 having a thickness of, for example 1 to 2 μm. This first layer is particularly suitable for adhesive bonding of components. This first layer 2 is also suitable as a base coating for a subsequent lacquer coating of the components. The second anodizing step involves the use of a second electrolyte that is a mixture of an organic and an inorganic acid. For example, a tartaric acid is mixed with a sulfuric acid. Exposing the compound to this second electrolyte results in a low porosity second anodizing layer 3 having a preferred thickness within the range of 2 to 4 μm. This layer is formed below the first layer 2 as explained above and can be subsequently densified, for example by a rolling operation whereby the desired corrosion resistance is further improved.

According to the invention it is possible to individually improve any one of the above mentioned three desirable characteristics, namely the corrosion resistance and the lacquer acceptance characteristic and the ability to increase an adhesive bonding strength. This improvement can be made by selecting the anodizing parameters such as the ratio of the acid mixtures in the electrolytes, the voltage and current density in the electrolyte baths, the duration of bath exposure, and the individual electrolyte concentration in each individual bath.

The anodizing steps are performed in each bath at temperatures within the range of 20 to 70° C. The anodizing is performed at a DC voltage profile as shown in FIG. 2 by the ramp voltage RV and the plateau voltage PV. The plateau voltage PV is within the range of 3 to 25 V. The time for increasing the ramp voltage is within the range of 30 seconds to 10 minutes while the duration of maintaining the plateau voltage PV is within the range of about 5 to 90 minutes. The concentration of the electrolytes in the first anodizing step which is the so-called PSA step is within the range of 50 to 250 g/l of phosphoric acid and within the range of 50 to 150 g/l of sulfuric acid. In the second anodizing step referred to as TSA step, the electrolyte concentrations in the second bath are as follows: 20 to 150 g/l of (L)+tartaric acid and 20 to 150 g/l of sulfuric acid.

In a preferred example embodiment the following anodizing steps were performed at the following parameters. This example embodiment of the two baths was suitable for any aluminum and aluminum alloys: First anodizing step (PSA-step) 125 g/l of H₃PO₄, 75 g/l of H₂SO₄. The anodizing takes place with a DC anodizing voltage profile in which the ramp voltage RV rises from 0 V to 15 V within 5 minutes followed by a plateau of 15 V for 15 minutes. The first anodizing is performed at room temperature.

Second anodizing step (TSA step) (80 g/l of (L)+tartaric acid, 40 g/l of H₂SO₄. The anodizing takes place at a DC anodizing voltage profile in which the ramp voltage RV rises from 0 to 13 V in 3.5 minutes. The 13 V plateau is maintained for 25 minutes and the bath temperature is 35° C.

The above example embodiment is particularly suitable for creating a surface morphology or texture which is substantially ragged or jagged. Such a surface texture is particularly suitable for adhesive bonding and lacquer adhesion. The second layer that is formed below or underneath the first layer has a high corrosion resistance. The first step that produces the first layer 2 is the PSA step. The second step that produces the layer 3 under the first layer is the TSA step.

Although the invention has been described with reference to specific example embodiments, it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims. It should also be understood that the present disclosure includes all possible combinations of any individual features recited in any of the appended claims. 

1. A method for anodizing a surface of structural components made of aluminum or aluminum alloys, said method comprising the following steps: a) first preparing an inorganic acid mixture of at least two inorganic acids as a first electrolyte, b) first exposing said surface to said first electrolyte to provide an anodized first surface coating, c) second preparing a further acid mixture of an organic acid and an inorganic acid as a second electrolyte, and d) second exposing said surface to said second electrolyte to provide a second surface coating.
 2. The method of claim 1, wherein said first preparing step is performed by mixing a phosphoric acid and a sulfuric acid to form said inorganic acid mixture as said first electrolyte.
 3. The method of claim 2, wherein said inorganic acid mixture contains between 50 to 250 g/l (gram per liter) phosphoric acid (H₃PO₄) and 50 to 150 g/l of sulfuric acid (H₂SO₄).
 4. The method of claim 3, wherein said inorganic acid mixture contains a mixture of 125 g/l of phosphoric acid (H₃PO₄) and 75 g/l of sulfuric acid (H₂SO₄).
 5. The method of claim 1, wherein said second preparing step comprises mixing a tartaric acid and a sulfuric acid to form said further acid mixture as an organic and inorganic acid mixture forming said second electrolyte.
 6. The method of claim 5, wherein said further acid mixture forming said second electrolyte contains 20 to 150 g/l (L(+)tartaric acid, and 20 to 150 g/l of sulfuric acid (H₂SO₄.)
 7. The method of claim 6, wherein said second electrolyte contains 80 g/l of sulfuric acid (H₂SO₄).
 8. The method of claim 1, further comprising performing said first and second exposing steps each at an anodizing DC voltage profile, and controlling said DC voltage profile such that an initial rising DC voltage forming a ramp voltage as a function of time, is followed by a constant DC voltage forming a plateau as a function of time.
 9. The method of claim 8, wherein said ramp voltage reaches said plateau voltage at a DC voltage within the range of 3 to 25 V DC.
 10. The method of claim 8, wherein said ramp voltage has a rise time within the range of 0.5 to 10 minutes.
 11. The method of claim 8, wherein said plateau voltage is maintained for a duration within the range of 5 to 90 minutes.
 12. The method of claim 8, further comprising performing said first exposing step at an anodizing DC voltage profile by controlling said anodizing DC voltage to increase from 0 to 15 V DC within 5 minutes and then holding said DC voltage at 15 V DC for 15 minutes.
 13. The method of claim 8, further comprising performing said second exposing step at an anodizing DC voltage, and controlling said anodizing DC voltage to increase from 0 to 13 V DC within 3.5 minutes and then holding said DC voltage at 13 V DC for 25 minutes.
 14. The method of claim 8, further comprising performing said first and second exposing step at a temperature within the range of 20 to 70° C. for anodizing.
 15. The method of claim 8, further comprising performing said first exposing step at room temperature for anodizing.
 16. The method of claim 8, further comprising performing said second exposing step at a temperature of 35° C. for anodizing.
 17. The method of claim 8, further comprising continuing said exposing steps until an anodized layer thickness is achieved within the range of 1 to 10 μm for each layer formed by said first and second exposing steps.
 18. The method of claim 8, wherein said inorganic acid mixture for said first exposing step contains between 50 to 250 g/l of phosphoric acid (H₃PO₄).
 19. The method of claim 8, wherein said further acid mixture of said inorganic acid and of said organic acid for said second exposing step contains between 20 to 150 g/l of L(+)tartaric acid.
 20. A structural component made of any one of aluminum and aluminum alloys comprising an anodized surface texture produced according to the steps of claim
 1. 21. A method for anodizing a surface of structural components made of aluminum or aluminum alloys, said method comprising the following steps: a) first preparing an acid mixture of an organic acid and an inorganic acid as a first electrolyte, b) first exposing said surface to said first electrolyte to provide a first surface coating, c) second preparing an inorganic acid mixture of at least two inorganic acids as a second electrolyte, and d) second exposing said surface to said second electrolyte to provide an anodized second surface coating. 